Location based services are today becoming more and more important for the cellular industry. The major driving force is emergency positioning, denoted E-911 positioning in North America. The accuracy requirements for E-911 positioning are quite stringent, which has lead to a technical solution with Assisted Global Positioning System (A-GPS) as the main positioning method. One or several fallback positioning methods are also normally implemented to cover up where A-GPS works less well, e.g., indoors. Common such methods include cell ID positioning, timing advance (TA) positioning, fingerprinting positioning as well as time difference of arrival methods in the uplink and downlink. These methods are reviewed below. Currently, with the emergence of A-GPS capable cell phones, commercial applications are expected to emerge at a larger scale. Such applications include e.g., personal navigation, friend and service finding, and gaming applications.
A-GPS Positioning
A-GPS positioning is an enhancement of GPS. An example of an A-GPS based positioning system is displayed in FIG. 1, such as might be implemented in a Wideband Code Division Multiple Access (WCDMA) system. In such systems, GPS reference receivers attached to a cellular communication system collect assistance data that, when transmitted to GPS receivers in terminals connected to the cellular communication system, enhance the performance of the GPS terminal receivers. Typically, A-GPS accuracy can become as good as 10 meters without differential operation. The accuracy becomes worse in dense urban areas and indoors, where the sensitivity is most often not high enough for detection of the very weak signals from the GPS satellites.
Cell ID Positioning
The cell ID positioning method determines the terminal location with cell granularity, by association of the cell ID to a geographical description of the cell. Standardization may not be finalized in LTE, however in WCDMA a polygon with 3-15 corners is used for this purpose.
TA Positioning
The TA positioning principle is depicted in FIG. 2. Briefly, the travel time of radio waves from the “eNodeB,” which is a type of cellular radio base station, to the terminal is measured. The distance from the eNodeB to the terminal can then be computed
  r  =      c    ⁢                  T        ⁢                                  ⁢        A            2      where TA is the timing advance value and where c is the speed of light.
The TA measurement alone defines a circle, or if the inaccuracy is accounted for, a circular strip around the eNodeB. By combining this information with the cell description, left and right angles of the circular strip can be computed. In particular, FIG. 2 illustrates cell identity positioning combined with TA, where the terminal position is determined as the intersection of the serving cell and the circular strip.
Fingerprinting Positioning
Another approach is provided by so called fingerprinting positioning. Fingerprinting positioning algorithms operate by creating a radio fingerprint for each point of a fine coordinate grid that covers the Radio Access Network (RAN). The fingerprint may e.g. consist of: the cell IDs that are detected by the terminal, in each grid point; quantized path loss or signal strength measurements, with respect to multiple eNodeBs, performed by the terminal, in each grid point—note that an associated ID of the RBS may also be needed; quantized TA, in each grid point—note that an associated ID of the eNodeB may also be needed; and radio connection information, like the radio access bearer (RAB).
Whenever a position request arrives to the positioning method, a radio fingerprint is first measured, after which the corresponding grid point is looked up and reported. This of course requires that the point is unique.
The database of fingerprinted positions (the radio map) can be generated in several ways. A first alternative would be to perform an extensive surveying operation that performs fingerprinting radio measurements repeatedly for all coordinate grid points of the RAN. The disadvantages of this approach include: the surveying required becomes substantial for small cellular networks; and the radio fingerprints are in some instances (e.g. signal strength and path loss) sensitive to the orientation of the terminal, a fact that is particularly troublesome for handheld terminals. For fine grids, the accuracies of the fingerprinted positions therefore become highly uncertain. This is unfortunately seldom reflected in the accuracy of the reported geographical result.
Another approach is to replace the fine grid by high precision position measurements of opportunity, and to provide fingerprinting radio measurements for said points. This avoids the above drawbacks, however algorithms for clustering of high precision position measurements of opportunity needs to be defined, and algorithms for computation of geographical descriptions of the clusters also need to be defined. These two problems are solved by previous patent applications on the “adaptive enhanced cell identity” (AECID) positioning method.
Time Difference of Arrival and Trilateration
The time difference of arrival (TDOA) method relies on measurements, typically on some pilot radio signal, from multiple base stations. The measurement is performed by means of correlation with the known signals of the base stations measured upon. The situation is depicted in FIG. 3.
Assuming that the measurements are successful for a number of cells, three of which are depicted in FIG. 3, the following relations between the measured TOAs in the terminal, the transmission times from the base stations (eNodeBs) and the distances between the terminals and the base stations follow:
                                                        t                                                T                  ⁢                                                                          ⁢                  O                  ⁢                                                                          ⁢                  A                                ,                1                                      +                          b              clock                                =                                    T              1                        +                                                                                                r                    1                                    -                                      r                    Terminal                                                                              /              c                                                          ⋮                                                                t                                                T                  ⁢                                                                          ⁢                  O                  ⁢                                                                          ⁢                  A                                ,                n                                      +                          b              clock                                =                                    T              n                        +                                                                                                r                    n                                    -                                      r                    Terminal                                                                              /                              c                .                                                          ⁢        Here tTOA,i, i=1, . . . , n denotes the measured time of arrivals (TOAs) in the terminal, Ti, i=1, . . . , n denotes the transmission times from the eNodeBs and c is the speed of light. The boldface quantities are the (vector) locations of the base stations and the terminal. bclock denotes the unknown clock bias of the terminal with respect to cellular system time. Now, in TDOA positioning, time of arrival differences with respect to the own site are formed according to
                              t                                    T              ⁢                                                          ⁢              D              ⁢                                                          ⁢              O              ⁢                                                          ⁢              A                        ,            2                          =                                            t                                                T                  ⁢                                                                          ⁢                  O                  ⁢                                                                          ⁢                  A                                ,                2                                      -                          t                                                T                  ⁢                                                                          ⁢                  O                  ⁢                                                                          ⁢                  A                                ,                1                                              =                                    T              2                        -                          T              1                        +                                                                                                r                    2                                    -                                      r                    Terminal                                                                              c                        -                                                                                                r                    1                                    -                                      r                    Terminal                                                                              c                                                      ⋮                                    t                                    T              ⁢                                                          ⁢              D              ⁢                                                          ⁢              O              ⁢                                                          ⁢              A                        ,            n                          =                                            t                                                T                  ⁢                                                                          ⁢                  O                  ⁢                                                                          ⁢                  A                                ,                n                                      -                          t                                                T                  ⁢                                                                          ⁢                  O                  ⁢                                                                          ⁢                  A                                ,                1                                              =                                    T              n                        -                          T              1                        +                                                                                                r                    n                                    -                                      r                    Terminal                                                                              c                        -                                                                                                                        r                      1                                        -                                          r                      Terminal                                                                                        c                            .                                          
In these n−1 equations, the left hand sides are known (with some additional measurement error), provided that the time of transmission differences (denoted the real time differences) can be measured. Further the locations of the base stations, ri, i=1, . . . , n, can be surveyed to within a few meters and thus are known as well. What remains unknown is the terminal location, i.e.,rTerminal=(xTerminalyTerminalzTerminal)T.In the more common case, a two dimensional positioning is performed and the unknown position is instead expressed asrTerminal=(xTerminalyTerminal)T.
It then follows that at least three time of arrival differences are needed in order to find a 3D terminal position and that at least two time of arrival differences are needed in order to find a 2D terminal position. This, in turn, means that at least four sites need to be detected for 3D terminal positioning and at least three sites need to be detected for 2D terminal positioning. In practice, accuracy can be improved if more measurements are collected and a maximum likelihood solution is introduced. There may also be multiple (false) solutions in cases where only a minimum number of sites are detected.
Angle of Arrival Positioning
Angle of arrival positioning exploits multiple antenna elements to measure the angle of arrival of radio waves impinging on said array. In the uplink it is easy to understand that angle of arrivals measured in non-colocated sites are needed to compute a position in the plane. This makes pure angle of arrival positioning a multi-cell technology, a fact that increases the complexity and cost of implementation significantly. Further, in rural regions base station geometry may not allow measurement in multiple eNodeBs.
Hence a base station may combine AoA with TA, in one cell. Since AoA and TA are essentially orthogonal direction-wise in the terminal position, the accuracy of such a method should be good, at least in situations where radio propagation is good, without too much multipath and non line of sight effects. This should be the case, for example, in rural areas without hills. The principle is depicted in FIG. 4.
Architectural Considerations—Single Cell vs. Multiple Cells
In LTE systems, eNodeBs communicate with each other over the X2 interface, and with terminals over the RRC interface; see FIG. 5, depicting an LTE RAN architecture. As for AoA positioning using uplink measurements, this means that signaling will be needed over X2 in the case of pure AoA positioning, whereas the combination with TA does not require this. It is however possible to combine AoAs from multiple base stations with TA as well.
Architectural_Considerations—Control Plane vs. User Plane
Positioning can be performed either over the control plane (CP) or the user plane (UP). In the first case, measurements performed in the UE need to be signaled over the RRC interface to the eNodeB, for further transfer to the positioning node. AoA (uplink) positioning does not require any signaling because AoA measurements are performed in the eNodeBs and because TA is available in the serving eNode as well.
User plane positioning is entirely different because, with user plane positioning, the terminal communicates directly with a positioning node external to the RAN, using communication that is transparent to the eNodeB. The current trend is towards more user plane positioning. For example, certain network operators, such as VERIZON, prefer use plane positioning for LTE.